An important process for removing hazardous hydrogen sulfide (H2S) from various waste gases, including gases produced during the refining of petroleum products, is known as the Claus process. It involves the following net reaction:H2S+½O2→H2O+S   (1)
Sulfur produced by the Claus process contains high levels of H2S (typically, 250 to 300 ppmw) which exists as both dissolved H2S and as H2Sx hydrogen polysulfides bound in liquid sulfur. The dissolved H2S separates from the sulfur readily; however, the H2S bound in the hydrogen polysulfides must be first decompose back into H2S and elemental sulfur.H2Sx→H2S+Sx-1 
This reaction is slow and accounts for the difficulty in degassing sulfur. The residence of time of liquid sulfur in conventional degassing processes can be several days.
The motivation to degas sulfur arrives from the toxicity, explosiveness, and corrosive nature of H2S. H2S is lethal at 600 ppmv, and is explosive at roughly 3.5% volume in air. Both of these conditions are of concern, especially during loading and unloading operations. The head space in a tank or tank truck can easily exceed the toxicity and explosive limit if the sulfur is not degassed.
Conventionally degassing takes place after the Claus process. Sulfur from the Claus process flows into a pit. Over time, as the sulfur cools somewhat and is agitated, H2Sx compounds decompose and form dissolved H2S and elemental sulfur. Desorbed H2S collects in the head or vapor space in the pit or vessel above the sulfur.
Despite the fact that both H2S and sulfur are flammable in air, the conventional industry practice is to use an air sweep of the sulfur pit vapor space to maintain the H2S level to well below the Lower Explosive Limit (LEL) of H2S. The Lower Explosive Limit for H2S is 3.85% at a storage temperature of 330° F. It is common industry practice to have sufficient sweep air to maintain a H2S concentration of less than 1% in the vapor space above the sulfur pit and thereby achieve a margin of safety.
Sweep air is typically drawn from the head space by a blower or a steam eductor. Such equipment is subject to fouling by crystalline sulfur.
Further, it is a disadvantage of modern, commercial degasification processes that they require large, complex and accordingly, expensive equipment. For example, in one process, known as the Shell process, degassing takes place in a storage tank or sulfur pit equipped with stripping columns, where liquid sulfur is vigorously agitated by bubbling air there through at atmospheric pressure. The stripping columns are open at the tops and bottoms to allow the sulfur to circulate at a rate of few hundred times per hour. The bubbling air, together with an additional flow of air, is then used as a low pressure sweep gas to displace the gases produced by the degasification process. The low pressure gases so produced are then fed to an incinerator where the H2S is oxidized to SO2 and released to the atmosphere. Depending on the design, a liquid or gaseous catalyst, such as ammonia, ammonium thiosulfate, urea, morpholine, or an alkanol amine may be added for accelerating the decomposition of the polysulfide to H2S and elemental sulfur.
In an alternative process, known as the D'GAASS process, degassing takes place in a vessel under pressure of at least 40 psig to 75 psig. Compressed air and high pressure sulfur are pumped to this pressure vessel. The pressure vessel contains a static mixing device which provides intimate contact between the two streams. The thus degassed liquid sulfur is discharged from the vessel and the air containing the H2S is discharged to an incinerator.
In another alternative process, known as the SNEA process, degassing takes place by repeated circulation and spraying the liquid sulfur into the sulfur pit. Release of dissolved H2S is achieved by spraying liquid sulfur through jets at a specific velocity. Ammonia, injected at the suction of the recirculation pump, is typically used as a catalyst. After the H2S gas is released, it is removed by a sweep gas and fed to an incinerator.
Both the stripping columns used by the Shell process and the circulation/spraying equipment used in the SNEA process are costly and require a large amount of space. The D'GAASS process ignores the requirement to have sweep air in the sulfur pit vapor space or safe operation of the sulfur pit.
Accordingly, there has existed a definite need for a degasification process that, not only effectively reduces the H2S content of liquid sulfur but, is simple, requires a minimum of space and is inexpensive and results in safe conditions.